Damp cellar

[Murmansk]

Someone has asked me to install a fan in a cellar that's been damp

The cellar is in an old Victorian building, there was a drain problem that's now been sorted but the ground is still quite damp following that work.

The cellar is about five metres wide and goes back about 15 metres, with rooms towards the back where the damp floor is. There are a couple of airbricks at the front but at the back there's not much air movement.

The owner wants me to install an extractor fan, plugged into a mains sockets and connected to a long length of tubing going back to the damp area via the ceiling and then taking the damp air out of the airbrick at the front.

My gut feeling is that this won't have much effect or at least that to have much effect we'd need a massively powerful fan and I'm wondering what people think and what alternatives there might be

I think an industrial strength dehumidifier might be better as that'd tend to "suck" the moisture out of the ground?

[Roger Mills]

I can't quite visualise the setup, and the relationship between the cellar and the "rooms".

But you somehow need to create a through draft through the whole cellar. This will be more effective than a fan.

My holiday flat is in a Victorian building which has a cellar. This has suffered from damp from time to time. The situation has been greatly improved since we propped the coal chute cover slightly open at one end of the cellar and fitted an additional air brick at the other end.

[Animal]

Fan effectiveness depends on relative temps, typically cellars are cool which really hampers fan drying. A domestic dehumidifier ought to be enough, works year round & doesn't waste a ton of heat/money

[Thomas Prufer]

Possibly, but then you would be going against the recommendations of the owner, and are "liable" for everything that could go wrong: electricity bill, emptying the dehumidifier, the smell, the weather, the lottery numbers, ... If you go with the wishes of the owner, you're home free:-)

Second, do the numbers, i.e. get a dewpoint app/calculator and plug some reasonably assumed numbers in: humidity and temp of the "fresh" air from outside, assume exhaust air to have the ambient inside temp and 100% rH, and look at the grams per cubic meter and the volume the fan will move. That'll give you a ballpark number, "liters/grams per day", which you can compare to a dehumidifer. Maybe assume a cheap clock timer to not run the fan at night, or such. Much depends on where the "fresh" air will come from.

I have found this to sometimes give surprising (to me) results, either way.

Thomas Prufer

[Jeff Gaines]

Unless they are relying on your professional expertise. If you go to a doctor with symptoms of something potentially serious and you say that you'll be happy if he prescribes white chocolate Magnums he/she will do everything to ensure you get proper treatment.

[Thomas Prufer]

Yes... but: "Someone has asked me to install a fan in a cellar that's been damp" could be read as "please do exactly as I wish, and don't argue about why or if it will work".

A little bit like installing a magnetic fuel efficiency booster in a car, or a HiFi-mains-filterthing, or similar snake oil -- would you do it if asked? (Though I think both the fan and the dehumidifier will dry the cellar, and both will work better than "doing nothing".)

I probably would let that depend on who was asking, and how:-)

Thomas Prufer

[The Natural Philosopher]

(Ordinary) fans do not work well in tubes.

It is surprising how little airchange you need to de humdify, rig up one temporarily to see if it will work.

Id be incline also to render the rear walls with a waterproof render

[Brian Gaff]

We used to stay in Guernsey in a flat which was on the side of a hill, this meant in effect that the flat was the basement on one side, but at ground level on the other. They had had fitted a row of vents in the back wall of the side which was underground, and had a vent going up and out of the top of the roof of the part above it on the up hill side. No ign of any damp which apparently the owners had issues with prior to the modification.

As you say it made a through draught. However a friend has a house with a basement very near the Thames and there is, apparently no easy fix for the problem there. Brian

[Jeff Gaines]

The legal profession lives on anything arguable :-)

[Colin Bignell]

I would be tempted first to hire a dehumidifier and try that. If the damp is a residual problem from an earlier drain problem, that might be all it needs to sort it out. If it doesn't, then would be the time to consider improving air flow. Depending upon the site, a vent pipe with a Savonius rotor extractor fan at the end might be better than an electric fan.

[Clive Arthur]

An old cellar with earth the other side of brick walls will be damp, but with adequate ventilation, not very. Ours had a coal chute and a small window-sized opening to a small excavation with a grating at ground level outside. I'm sure there's a word for that. Would these 'rooms' of which you write once have had similar?

We used ours for the washing machine and dryer, and they didn't rust away, but you wouldn't want to keep anything too corrosion sensitive there.

Dehumidifying may help for a short term problem, but you'll never dry it out completely. Tanking is expensive, and there can (rarely) be flotation issues.

[Murmansk]

OP here

The cellar is largely one big room but at the back there are walls making a couple of smaller rooms which together probably make up about 20% of the floor area.

I reckon an industrial dehumidifier will draw the wetness out of the floor from where the drain issue was and, given that the drain has been mended, it should stay dry as it's not inherently damp.